1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
As materials of semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
In many cases, transistors that use amorphous silicon, which can be manufactured using the established manufacturing technique, are utilized in display devices; however, the transistors that use amorphous silicon have low field-effect mobility, having difficulty in achieving high definition, low power consumption, and the like of display devices.
Further, the transistors that use amorphous silicon also have a problem of severe electrical characteristic deterioration (low reliability), which occurs with the temperature variation or repetitive operation.
As the integration becomes higher obeying the scaling law and the circuit structure becomes complex, a problem of an increase in power consumption becomes more significant in semiconductor devices (such as semiconductor memory devices) utilizing transistors that use single crystal silicon and have high field-effect mobility.
It is known that a transistor that uses an oxide semiconductor has higher field-effect mobility than a transistor that uses amorphous silicon. In addition, the oxide semiconductor can be easily deposited on a mother glass which has a large area by a sputtering method or the like, and thus, application of the oxide semiconductor to display devices has been actively examined.
On the other hand, it is pointed out that aluminum oxide having high resistance is produced when the oxide semiconductor is directly connected to an aluminum-based alloy wiring, thereby increasing contact resistance (see Patent Document 1).
Even in the case of using a metal that is relatively unlikely to be oxidized or a metal, an oxide of which has a conducting property, a metal oxide having high resistance is often produced at the interface between the metal and an oxide semiconductor by heat treatment or the like in a later step; accordingly, contact resistance is often increased.
There is a problem in that such high contact resistance between a metal and an oxide semiconductor causes a reduction in on-state characteristics of a transistor.
As a method for reducing the contact resistance, a technique in which a buffer layer having low resistance is provided between an oxide semiconductor and a metal is disclosed. In addition, an oxide semiconductor containing nitrogen, which serves as a buffer layer, is disclosed (see Patent Document 2).